Adsorption of heavy metals in waste water

ABSTRACT

A method and adsorbent composition for removing heavy metals from contaminated water, including mixing a water having a concentration of one or more heavy metals with an adsorbent including granules of a mixture of 3.33 wt % bentonite clay and a siwak stick powder; and collecting water having a reduced concentration of the heavy metal(s).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 12/835,888, filed on Jul. 14, 2010, now U.S. Pat. No.8,394,739, the contents of which are incorporated herein by reference intheir entirety.

FIELD OF THE INVENTION

The invention relates to an adsorbent for removing heavy metals fromcontaminated water, and more particularly, to a process fordecontaminating a heavy metal contaminated stream of water.

BACKGROUND OF THE INVENTION

In the area of water treatment, such as ground water or industrial wastewater treatment, there is an ever-increasing need to remove undesirableand even toxic contaminants, particularly heavy metal contaminants, fromwater. Many industrial processes utilize aqueous solutions of heavymetals, such as lead in the manufacture of batteries, and chromium orcopper in electroplating solutions.

Unfortunately, the removal of such heavy metals from the aqueoussolutions used in these processes has proven to be not only difficultbut expensive. Prior art processes have utilized different types ofadsorbents, such as activated carbon, activated sludge, various types ofnatural clays, carbon aerogels, coirpith carbon, natural zeolites andeven date pits. Likewise, heavy metal removal can be accomplishedthrough expensive ion exchange resins.

U.S. Pat. No. 4,133,755 to Tarao et al., incorporated by referenceherein in its entirety, discloses agents for removing heavy metalscomprising a composition consisting mainly of a dithiocarbamatebond-containing low molecular weight compound, amorphous silica andactivated carbon powder, granulated with a vinyl acetate polymer binderand clay as a thixotropic excipient. Tarao et al. disclose packing thegranulated materials into columns for treating mercury-contaminatedwaste water.

U.S. Pat. No. 5,667,694 to Cody et al., incorporated by reference hereinin its entirety, discloses a process for removing dissolved heavy metalsincluding lead and radioactive contaminants from contaminated aqueoussystems including aqueous soil systems. An organically modified smectiteclay, or organoclay, is used to treat these systems. Organoclays are thereaction product of smectite clays and quaternary ammonium compounds.The organoclay is brought in contact with system to be treated where itsorbs the heavy metal in the aqueous system onto the organoclay whichsorbed complex is then removed by a variety of methods includingflotation and air sparging.

U.S. Pat. No. 5,256,615 to Oomura et al., incorporated by referenceherein, discloses a granular inorganic ion exchanger which is obtainedby firing at 400° C. or higher a granular molded product of a mixture ofa metal alkoxide such as Si(OMe)₄ or hydrolyzate thereof, a clay mineralsuch as sepiolite and an inorganic ion exchanger such as antimonypentoxide and which has mechanical strength and heat resistance withoutlosing its inherent ion exchangeability.

World Patent Publication No. WO 00/72958 to Payzant et al., incorporatedby reference herein in its entirety, discloses a networked polymer/clayalloy produced from a monomer/clay mixture comprising a monomer, across-linking agent and clay particles. The clay is chemicallyintegrated with the polymer such that, on exposure to water, thenetworked polymer/clay alloy swells with substantially no clayseparating from the alloy.

However, none of the above-discussed references discloses or suggests arelatively inexpensive but highly effective adsorbent composition forremoval of heavy metal contaminants from contaminated water streams.Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove.

SUMMARY OF THE INVENTION

In accordance with aspects of the invention, the invention is directedto an adsorbent composition for metal ions, comprising a granulatedmixture of 3.33 wt % Saudi bentonite clay and a 63.33 wt % Saudi siwakstick powder.

In accordance with aspects of the invention, the invention is directedto a method for removing heavy metals from contaminated water,comprising mixing a water having a concentration of one or more heavymetals with an adsorbent composition comprising granules of a mixture of3.33 wt % bentonite clay and a 63.33 wt % Saudi siwak stick powder; andcollecting water having a reduced concentration of the heavy metal(s).

In aspects of the invention, an adsorbent composition for metal ions isprovided, comprising a granulated mixture of 3.33 wt % clay and a siwakstick powder. The clay has ion-exchange capacity. The clay is bentoniteclay. Providing about 33.3 wt % of a triethylamine (TEA). The size rangefrom about 0.125 mm to about 0.25 mm.

In aspects of the invention, a method is provided for removing heavymetals from contaminated water. The method comprises mixing a waterhaving a concentration of one or more heavy metals with an adsorbentcomposition comprising granules of a mixture of 3.33 wt % bentonite clayand a siwak stick powder. The method further comprises collecting waterhaving a reduced concentration of the one or more heavy metals. Theprocess is conducted at a pH>4. The mixing step is conducted at a pHbetween about 4 and 5. The adsorbent composition is a mixture of about3.33 wt % clay in a siwak stick powder. The one or more heavy metals isselected from the group consisting of lead, chromium, copper, zinc,cadmium and combinations thereof. A concentration of the one or moreheavy metals in the water is between about 50 ppm and about 900 ppm. Theone or more heavy metals is lead, and the concentration of lead in thewater is reduced by at least about 70%.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

The FIGURE shows the adsorption system including a batch adsorption unitfor removing heavy metals from contaminated waste water in accordancewith aspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an adsorbent composition for removing heavymetals from contaminated water, and more particularly, to a process fordecontaminating a heavy metal contaminated stream of water.

The FIGURE shows the adsorption system including a batch adsorption unitfor removing heavy metals from contaminated waste water in accordancewith aspects of the invention. In particular, the adsorption system 5includes a wastewater tank 10. The wastewater tank 10 can include heavymetals as discussed below. A pump 20 pumps the wastewater in thewastewater tank 10 to a batch absorption unit 30. As discussed in detailbelow, the heavy metals in the wastewater will be removed in the batchabsorption unit 30. Prior to use, the remaining water will be filteredby a filter 40. The filter 40 can be any well known filter used toremove heavy metals such as, for example, wastewater ion exchangefilters.

Salvadora persica is a tree from which siwak stick is made from itstwig/root and is commonly known as Arak tree. This tree is naturallygrowing in Jazan, Saudi Arabia. Siwak in the form of stick was crushedand milled to a particle size below 125 mesh (particles sized betweenabout 0.125 mm to 0.25 mm). Then, siwak stick powder was used to removeheavy metals from waste water. The aim of this research was to find outthe naturally available and low cost material for the treatment of wastewater. There was no previous study carried out to use siwak stick powderas adsorbent for adsorption of metal ions from wastewater. Siwak stickhas an inherent ion-exchange capacity at the pH range (3-5) where theexperiment is performed. For example, siwak stick contains manyfunctional groups (as found by FT-IR, Table 1) such as carboxylicgroups. Since the solution pH affects the charge on the functionalgroups, therefore functional groups such as carboxylate are protonatedat low pH values (In “Equilibrium Study of Adsorption of Cobalt ionsfrom Wastewater using Saudi Roasted Date Pits” Research Journal ofEnvironmental Toxicology, 1:12 (2010), by S. A. Al-Jlil). As the pHincreased, deprotonation of the functional groups might have occurredand become available for metal ion binding. The metal ions hasattraction force with the negative charge on the functional groups ofsiwak stick powder, such as carboxylate at alkaline solution when the pHof solution was high. Therefore, in the pH range (3-5) where experimentsfor the invention were performed, the electrostatic forces between themetal ions and the functional groups are not present and the metal ionscan exchange with the proton of the functional groups by an ion exchangeprocess. Some main functional groups in Saudi siwak stick powder areprovided in Table 1. The surface area and pore characteristics of thesiwak stick powder as an adsorbent are given in Table 2 and chemicalanalysis by XRF is given in Table 3.

TABLE 1 Main functional groups in Saudi siwak stick powder Observed band(cm⁻¹) Functional group 3488-3100 O—H N—H 2800-2900 C—H 1600-1740 C═OCarbonyl 1000-1200 C—O

TABLE 2 Surface area and pore characteristics of Saudi siwak stickpowder Element siwak stick powder BET surface area (m²/g) 0.6933 Porevolume (p/po = 0.97) (cm³/g) 0.001788 Average pore width (Å) 103.144

TABLE 3 Chemical analysis of Saudi siwak stick powder by XRF ElementComposition (%) S 23.0752 Cl 11.195 K 10.02 Ca 55 Fe 0.492 Zn 0.108 Br0.0998

Naturally occurring clay has been known for use in treating watercontaminated with heavy metals, since some such clays have an inherention-exchange capacity. For example, clay minerals have been found to bein the form of layered molecular structures, often having variableamounts of iron, magnesium, alkali metals, alkaline earths and othersuch cations sandwiched between the layers, which cations can exchangewith heavy metal cations in aqueous solution.

In “Saudi Arabian clays for lead removal in wastewater”, Applied ClayScience, 42:671-674 (2009), by S. A. Al-Jlil and F. D. Alsewailem,incorporated by reference herein in its entirety, several naturallyoccurring Saudi Arabian clay materials were tested for lead adsorptionefficiency from wastewater in batch processes. In some experiments,pretreatment of the clay with hydrochloric acid was found to increaselead adsorption efficiency, while other clays performed better in theabsence of pretreatment. Additionally, adsorption efficiency wasdemonstrated to increase with increasing pH of the lead-contaminatedwater solution.

Therefore, clays can be used to enhance the adsorption capacity andremoval efficiency of natural material such as our new material siwakstick powder. As such, bentonite clay has typically been added in only3.33 wt %, since the adsorption capacity of siwak stick powder wasapproximately 20.35 mg/g. So, by adding bentonite clay, the overallefficiency in heavy metal removal from the process stream was increased(up to 70.1%).

In an effort to increase the siwak stick powder capacity as adsorbentfor adsorption of heavy metals from waste water, the research leading tothe present invention has discovered a new adsorbent composition forheavy metal ions which are dissolved in aqueous media and betteradsorption capacity and removal efficiency than siwak stick powderalone. The enhancement of adsorption capacity by preparing the siwakstick/clay composite is 33.15% as compare to siwak stick powder aloneand the enhancement in removal efficiency is 16.2%. In addition to thesurface area and pore volume as shown in the below Table:

Enhance- Enhancement Enhancement Enhance- ment BET of adsorption inremoval Enhance- ment in Maximum Removal surface capacity % efficiency %ment in in pore average capacity efficiency area by preparing bypreparing surface volume pore Type (mg/g) (%) (m²/g) composite compositearea % % width % siwak 20.35 58.81 0.6933 33.15 16.11 86.3 87.8 10.96stick powder siwak 30.44 70.1 5.0515 stick- bentonite clay composite

In embodiments, an adsorbent composition for metal ions is disclosedcomprising a granulated mixture of 3.33 wt % bentonite clay and a siwakstick powder.

In order to enhance such excellent combination of clay particles withinthe siwak stick powder, it can be advantageous to incorporate betweenabout 33.3 wt % a triethylamine (TEA), with the clay and siwak stickpowder upon mixing.

One manner of making the new adsorbent composition is to mill clay to asize below about 125 mesh and wash the clay particles with distilledwater several times to remove impurities. The clay is then dried in avacuum oven, e.g., overnight. The dried clay is composited with siwakstick powder, triethylamine, the blended is placed in an autoclave andthen heated in an oven at a temperature about 200° C., even above 300°C., or even above 400° C. to make a paste. The paste is collected after24 hours, dried and subsequently ground to granules of different sievesizes, ranging for example from about 0.125 mm to about 0.25 mm.

The granulated adsorbent composition is then used as adsorbent for theremoval of heavy metal ions, such as Cu, Cr and Pb, from industrialwastewater to obtain clean water for the development of landscape andindustrial cooling. It has been observed that the adsorption capacitywas enhanced by more than 70%, which is much less expensive as comparedto other conventional technologies used for wastewater treatment for theremoval of heavy metals.

The removal method can be enhanced by mixing the heavy-metalcontaminated water with adsorbent composition. In embodiments, thesolution being treated is maintained at a pH=4.5, and is treated for atleast about 30 minutes at a room temperature or above.

The removal method, in embodiments, utilizes clay particles which have anaturally-occurring ion-exchange capacity, such as a bentonite clay. Theremoval method is effective in removing heavy metals including lead,chromium, copper, zinc, cadmium and combinations thereof. It has beenfound that when the heavy metal is lead the concentration of lead in thewater is reduced by more than about 70%.

EXAMPLE 1

Saudi siwak in the form of stick was crushed and milled to a particlesize below 125 mesh (particles sized between about 0.125 mm to 0.25 mm),washed with distilled water several times and dried in a vacuum ovenover night. 0.25 gram of adsorbent particles were mixed in the batchabsorption unit 30 as shown in the FIGURE, then 692.24 ppm Pb solutionwas mixed at 250 RPM with the adsorbent at 25° C. temperature. Thesamples were collected after thirty minutes. The lead content of thewater was tested by atomic absorption spectroscopy and determined to beless than 285.17 ppm, the removal efficiency being greater than 58.81%.

EXAMPLE 2

Bentonite clay in the form of soft rocks was received and milled to aparticle size below 125 mesh, washed with distilled water several timesand dried in a vacuum oven over night. The dried clay particles werethen dry blended with siwak stick powder (63.33%) and triethylamine atconcentrations of about 3.33 wt % clay particles and about 33.3 wt %triethylamine. The mixture was then put in an autoclave, then put in anoven at a temperature of about 200° C. to make a paste. The paste wasdried, ground into particles sized between about 0.125 mm to 0.25 mm.0.25 gram of adsorbent particles were mixed in the batch absorption unit30 as shown in the FIGURE, then 869 ppm Pb solution was mixed at 250 RPMwith the adsorbent at 25° C. temperature. The samples were collectedafter thirty minutes. The lead content of the water was tested by atomicabsorption spectroscopy and determined to be less than 260 ppm, theremoval efficiency being greater than 70%.

EXAMPLE 3

The dried clay particles of Example 2 were dry blended with siwak stickpowder particles 63.33 wt % and triethylamine at concentrations of about3.33 wt % clay particles and about 33.3 wt % triethylamine, relative tothe weight of the entire composition. The mixture was then put in anautoclave then heated in the oven at a temperature of about 200° C. Thepaste collected was dried, ground into particles sized between about0.125 mm to 0.25 mm. 0.25 grams of adsorbent particles were combinedwith 12.5 mL water contaminated with 869 ppm Pb, and stirred in a batchprocess for about 30 minutes at a 25° C. temperature. After stirring wascomplete, the lead content of the water was tested and determined to beless than 260 ppm, the removal efficiency being greater than about 70%.

The foregoing examples have been provided for the purpose of explanationand should not be construed as limiting the present invention. While thepresent invention has been described with reference to an exemplaryembodiment, changes may be made within the purview of the appendedclaims, without departing from the scope and spirit of the presentinvention in its aspects. Also, although the present invention has beendescribed herein with reference to particular materials and embodiments,the present invention is not intended to be limited to the particularsdisclosed herein; rather, the present invention extends to allfunctionally equivalent structures, methods and uses, such as are withinthe scope of the appended claims.

What is claimed:
 1. A method for removing heavy metals from contaminatedwater, comprising: mixing a water having a concentration of one or moreheavy metals with an adsorbent composition comprising granules of amixture of 3.33 wt % bentonite clay and a siwak stick powder; andcollecting water having a reduced concentration of the one or more heavymetals.
 2. The method of claim 1, wherein the process is conducted at apH>4.
 3. The method of claim 2, wherein the mixing step is conducted ata pH between about 4 and
 5. 4. The method of claim 2, wherein the mixingstep is conducted at a pH of 4.5.
 5. The method of claim 1, wherein theadsorbent composition is a mixture of about 3.33 wt % clay in the siwakstick powder.
 6. The method of claim 1, wherein the one or more heavymetals is selected from the group consisting of lead, chromium, copper,zinc, cadmium and combinations thereof.
 7. The method of claim 1,wherein a concentration of the one or more heavy metals in the water isbetween about 50 ppm and about 900 ppm.
 8. The method of claim 7,wherein the one or more heavy metals is lead, and the concentration oflead in the water is reduced by at least about 70%.
 9. The method ofclaim 1, wherein the clay is milled to a size below about 125 mesh andwashed in distilled water.
 10. The method of claim 9, wherein the clayis dried in a vacuum oven.
 11. The method of claim 10, furthercomprising blending the clay with the siwak stick powder andtriethylamine (TEA).
 12. The method of claim 11, wherein the clay, thesiwak stick powder and the TEA are blended and heated to at least 200°C. to make a paste.
 13. The method of claim 12, wherein the blend isheated to about 400° C.
 14. The method of claim 12, wherein the paste isdried and ground to granules of different sieve sizes ranging from about0.125 mm to about 0.25 mm.
 15. The method of claim 1, wherein the siwakstick powder is crushed to a particle size of between about 0.125 mm toabout 0.25 mm.
 16. The method of claim 1, further comprising providingabout 33.3 wt % of a triethylamine (TEA) with the clay and the siwakstick powder.
 17. A method comprising: milling bentonite clay to aparticle size below 125 mesh; washing the milled bentonite clay withdistilled water; drying the washed and milled bentonite clay in avacuum; dry blending the dried clay particles with siwak stick powderand triethylamine at concentrations of about 3.33 wt % clay particlesand about 33.3 wt % triethylamine; making a paste of the mixture of thesiwak stick powder, triethylamine and bentonite clay by placing themixture in an oven to about 200° C.; drying the paste to form anadsorbent composition; ground the paste into particles sized betweenabout 0.125 mm to 0.25 mm. mixing a water having a concentration of oneor more heavy metals with the adsorbent composition; and collectingwater having a reduced concentration of the one or more heavy metals.